As one of devices for performing observation and measurement of a sample using an electron beam, there exists a scanning electron microscope (SEM). The SEM acquires a two-dimensional image reflecting a surface configuration of a sample, by irradiating the sample with an electron beam and by detecting electrons emitted from the sample in the form of a signal.
The electron beam arrives at the sample and the arrived electron beam causes atoms on the sample surface to be excited, which emits a signal electron having a low energy. The signal electron is referred to as a secondary electron. When an electron beam is applied on an edge part of such a sample having irregularities as a semiconductor circuit pattern, this causes an edge effect to generate secondary electrons and increase the amount of such electrons, which results in formation of an image a contrast dependent on the irregularities.
On the other hand, an electron, again emitted from the sample surface while the electron beam is scattered in the sample, is referred to as a reflected electron. Such reflected electrons are signal electrons having high energies and form an image having a contrast not depending upon the irregularities of the sample but depending upon the composition of the sample.
In these years, as a process for a semiconductor, a magnetic head or the like is more complicated, and such measurement of a low-step sample as not having irregularities on the observation surface of a sample are more required. In such a case, the amount or magnitude of detected secondary electron signal is reduced. To avoid this, reflected electrons in place of secondary electrons are detected, and the signals and images are synthesized to reinforce information on edge and contrast, thus enabling enhancement of the image quality and the length measurement accuracy. The aforementioned method is valid, because, when the irradiation of the electron beam in the SEM observation causes the surface to be charged to several and several tens of volts (v), and even when this forms a barrier for the secondary electrons, reflected electrons can be detected.
There is disclosed in PATENT LITERATURE 1 a technique in which, when a reflected electron is detected and amplified, scanning conditions for an electron beam applied onto a sample before change of a magnification are set to be nearly the same as scanning conditions after the magnification change so as to form a suitable reflected electron image regardless of the magnification change. PATENT LITERATURE 2 explains a method by which, after a secondary electron image is selectively displayed and then adjusted in magnification and so on, the brightness and contrast of the secondary electron image are changed and adjusted operatively with the brightness and contrast of the secondary electron image of a reflected electron image not selectively displayed yet, and then observation is made by switching the display to the reflected electron image. PATENT LITERATURE 3 discloses the effect that, in order to detect and process a reflected electron signal or a secondary electron signal and an X-ray signal to obtain an integrated signal having a high contrast and signal/noise ratio (S/N ratio), respective magnifications, biases and weighting proportions are controlled for addition and division processing.